1. Field of the Invention
The present invention is directed to a method and system for preservation of sperm and other biological samples during freezing when cooling by placing into a cryogen, for example, immersing into liquid nitrogen for cryopreservation.
2. Description of Related Art
Sperm and other biological samples are frozen by slow controlled rate freezers using complex multiple sequential surrounding environments, such combinations of liquid nitrogen vapor and injections of liquid nitrogen. Sperm has also been cooled and frozen in a static liquid nitrogen vapor in a container, but the cooling rate control has been roughly by the height of the sperm straws to the surface of liquid nitrogen. Current methods also require sorting and packaging of the frozen samples inside the liquid nitrogen vapor or liquid nitrogen in a post freezing process, a process which imposes much additional work burden, risk of harm to personnel, and risk of decrease sperm quality during the process.
A high rate flash-freezing process known as vitrification has also been used, wherein special cryoprotectants are added to the sperm or other biological samples to decrease the freezing temperature and increase the viscosity, so that instead of crystallizing, the syrupy solution becomes an amorphous ice. Rather than a phase change from liquid to solid by crystallization, the amorphous state is like a “solid liquid”, and the transformation is over a small temperature range described as the “glass transition” temperature. Such processes are complex and often result in low quality sperm after thawing because of cooling rate and other problems.